The present invention relates to an exhaust gas purification apparatus and process for an internal combustion engine.
Japanese Patent No. 2600492 discloses one type of exhaust gas purification apparatus for an internal combustion engine, which is equipped with a NOx trap catalyst in an exhaust passage of the engine. The NOx trap catalyst traps thereon NOx in exhaust gas when an air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is lean, and reductively purifies the trapped NOx when the exhaust gas air-fuel ratio is richer than a stoichiometric air-fuel ratio. Thus, the above exhaust gas purification apparatus performs, at a purification timing, a so-called xe2x80x9crich spike operationxe2x80x9d for temporarily controlling the exhaust gas air-fuel ratio to a rich air-fuel ratio and thereby purifying NOx trapped on the NOx trap catalyst.
In the above exhaust gas purification apparatus, the rich spike operation is performed by introducing a reducing agent (HC, CO) to the NOx trap catalyst and causing a reaction between NOx and the reducing agent on the NOx trap catalyst in a reductive atmosphere.
However, there is a case that oxygen remains in the exhaust gas even when the air-fuel ratio of the exhaust gas is made rich. In such a case, the reaction between NOx and the reducing agent (HC, CO) does not occur unless the oxygen is consumed through the oxidation of HC and CO so as to create an adequate reductive atmosphere around the NOx trap catalyst. The reaction between NOx and the reducing agent is more unlikely to occur even at the same exhaust gas air-fuel ratio, as the exhaust gas contains a higher concentration of oxygen. Namely, NOx reduction rate per unit time decreases with increase in the concentration of oxygen in the exhaust gas flowing into the catalyst per unit time. For this reason, the above exhaust gas purification apparatus cannot maintain sufficient NOx purification performance and thereby causes a deterioration in automotive emissions when the concentration of oxygen in the exhaust gas is relatively high.
It is therefore an object of the present invention to provide an apparatus and process for purifying exhaust gas from an internal combustion engine with the use of a NOx trap catalyst, which can achieve and maintain sufficient NOx purification performance without being influenced by oxygen in the exhaust gas.
According to a first aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, comprising: a NOx trap catalyst disposed in an exhaust passage of the internal combustion engine, the NOx trap catalyst being capable of trapping thereon NOx in exhaust gas when an air-fuel ratio of the exhaust gas is lean and reductively purifying the trapped NOx when the air-fuel ratio of the exhaust gas is rich; and a control unit configured to: decide on a purification timing at which NOx trapped on the NOx trap catalyst is purified; determine a space velocity of the exhaust gas; at the purification timing, select either one of a first air-fuel ratio control mode for controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio with no need to decrease a concentration of oxygen in the exhaust gas and a second air-fuel ratio control mode for not only controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio but also making the concentration of oxygen in the exhaust gas lower than that of the first air-fuel ratio control mode, according to the determined space velocity; and control the air-fuel ratio of the exhaust gas in the selected air-fuel ratio control mode, thereby purifying NOx trapped on the NOx trap catalyst.
According to a second aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, comprising: a NOx trap catalyst disposed in an exhaust passage of the internal combustion engine, the NOx trap catalyst being capable of trapping thereon NOx in exhaust gas when an air-fuel ratio of the exhaust gas is lean and reductively purifying the trapped NOx when the air-fuel ratio of the exhaust gas is rich; and a control unit having: means for deciding on a purification timing at which NOx trapped on the NOx trap catalyst is purified; means for determining a space velocity of the exhaust gas; means for selecting, at the purification timing, either one of a first air-fuel ratio control mode for controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio with no need to decrease a concentration of oxygen in the exhaust gas and a second air-fuel ratio control mode for not only controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio but also making the concentration of oxygen in the exhaust gas lower than that of the first air-fuel ratio control mode, according to the determined space velocity; and means for controlling the air-fuel ratio of the exhaust gas in the selected air-fuel ratio control mode, thereby purifying NOx trapped on the NOx trap catalyst.
According to a third aspect of the present invention, there is provided an exhaust gas purification process for an internal combustion engine with the use of a NOx trap catalyst, the NOx trap catalyst being disposed in an exhaust passage of the engine and capable of trapping thereon NOx in exhaust gas when an air-fuel ratio of the exhaust gas is lean and reductively purifying the trapped NOx when the air-fuel ratio of the exhaust gas is rich, the process comprising: deciding on a purification timing at which NOx trapped on the NOx trap catalyst is purified; determining a space velocity of the exhaust gas; at the purification timing, selecting either one of a first air-fuel ratio control mode for controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio with no need to decrease a concentration of oxygen in the exhaust gas and a second air-fuel ratio control mode for not only controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio but also making the concentration of oxygen in the exhaust gas lower than that of the first air-fuel ratio control mode, according to the determined space velocity; and controlling the air-fuel ratio of the exhaust gas in the selected air-fuel ratio control mode, thereby purifying NOx trapped on the NOx trap catalyst.
According to a fourth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, comprising: a NOx trap catalyst disposed in an exhaust passage of the internal combustion engine, the NOx trap catalyst being capable of trapping thereon NOx in exhaust gas when an air-fuel ratio of the exhaust gas is lean and reductively purifying the trapped NOx when the air-fuel ratio of the exhaust gas is rich; and a control unit configured to: decide on a purification timing for reductively purifying NOx trapped on the NOx trap catalyst; and at the purification timing, selectively switch between a first air-fuel ratio control mode for controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio and a second air-fuel ratio control mode for not only controlling the air-fuel ratio of the exhaust gas to a rich air-fuel ratio but also making the concentration of oxygen in the exhaust gas lower than that of the first air-fuel ratio control mode, wherein the first air-fuel ratio control mode is selected upon judging a concentration of oxygen in the exhaust gas flowing into the NOx trap catalyst per unit time as being high, and the second air-fuel ratio control mode is selected upon judging the concentration of oxygen in the exhaust gas flowing into the NOx trap catalyst as being low.